An electrostatic recording film in which an electroconductive layer and an insulating layer are provided on an insulating film in this sequence is known.
In general, electrostatic recording is done in such a manner that a recording voltage is applied to a multi-pin electrode head (hereinafter, referred to as the pin electrode) to cause an aerial discharge in a narrow space (hereinafter, referred to as the gap) between the pin electrode and the insulating layer of the electrostatic recording film, whereby an electrostatic latent image is formed on the surface of the insulating layer, followed by developing the electrostatic latent image with a toner to thereby form a visible image.
In order to obtain a sharp image in the electrostatic recording system, it is necessary to control the gap in a predetermined range deviating from the Paschen's curve. For this purpose, such a system is generally employed that insulating spacer grains are added to give a suitable roughness to the insulating layer, and the pin electrode is contacted to the insulating layer to thereby control the gap in a prescribed range. It is known that in the above electrostatic recording film, a sharp image cannot be obtained without adding the insulating spacer grains, while it is known that imperfect grounding (earthing) of the electroconductive layer causes fogging.
In the electrostatic recording film using an insulating film, it is impossible to ground the film side of the insulating film, whereas in a conventional electrostatic recording paper, it is possible to ground the paper side of the electroconductive paper. In order to solve this problem in the electrostatic recording film, a portion of the electroconductive layer (usually, the end portion thereof) is exposed, or the exposed portion is coated with an electro-conductive paint such as a carbon paint to provide a grounding electrode. However, this results in decrease in manufacturing efficiency due to more time necessary to provide an exposed portion on the electroconductive layer according to the widths of various products, and an increased production step for coating an electroconductive paint. To cope with this problem, it is proposed in JP-A-61-213851 (the term "JP-A" as used herein refers to a published unexamined Japanese patent application) that electroconductive powders including metals such as Fe, Cu, Ni and Ag, alloys such as stainless steel and Ni-Cr alloy, metal oxides such as tin oxide, and metal compounds such as copper iodide, are introduced into the insulating layer, wherein the weight ratio of high polymeric binders to the electroconductive powders ranges from 100/0.1 to 100/10. In such an electrostatic recording film, however, while fog decreases, partial broadening of lines (hereinafter, referred to as pepper speck) and scratchwise dropout of images in the direction parallel to a recording electrode rather increase and make it impossible to use the film for drawings in CAD where precise drawing is required.
Further, there is the problem that the electro-conductive powders are liable to damage the recording electrode. To solve this problem, it is proposed in JP-A-2-83547 that carbon black, metals such as Fe and electroconductive grains such as tin oxide are introduced into an insulating layer, wherein the weight ratio of the high polymeric binders to the electroconductive grains is 100/0.0001 to 100/0.01 and that of the insulating spacer grains to the electroconductive grains is not more than 1000/5. In such an electrostatic recording film, while fog, pepper speck and scratchwise image dropout decrease without fail, they have not yet perfectly been prevented from causing and therefore, the improvement in this matter is strongly demanded.
In addition, a larger specific gravity causes the electroconductive powders to settle down in a coating solution. Prevention of settling necessitates a larger specific gravity and viscosity of the coating solution, which cause another problem that the larger specific gravity and viscosity deteriorates high speed coating.